[js] Couple of additional checks for module existence

lib/modules/database/index.js

@@ -211,7 +211,7 @@ export default class Database extends Base {
 }
 
 export const statics = {
-  ServerValue: {
+  ServerValue: FirebaseDatabase ? {
     TIMESTAMP: FirebaseDatabase.serverValueTimestamp || { '.sv': 'timestamp' },
-  },
+  } : {},
 };